Particulate matter distribution systems are generally associated with conveying a mass flow of finely rendered solid particles from a common source to multiple destinations, preferably providing equal amounts of the particulate matter to each destination. One method of conveying the particulate matter includes the provision of pressurized air. In one application, a pneumatic distribution system includes a number of pathways or conduits for continuously conveying finely pulverized coal to respective burners of a furnace, i.e., a coal fired furnace. A substantially balanced flow of coal particles is highly desirable, as an imbalanced flow of coal particle fuel to the burners can result in significant emissions of nitrogen oxides (NOx) and carbon monoxide (CO), having adverse environmental consequences. In addition, the imbalance can produce a significant variation in the absorptions throughout the boiler surfaces located after the furnace. This imbalance causes additional steam temperature variations among the superheater and reheater tubes which can reduce the life cycle of the tube material and produce lower overall efficiencies.
However, continuously balanced particulate flow is extremely difficult to achieve, and conventional techniques employed not only fail to provide balanced particulate flow, but also decrease the efficiency of the conveyance system. For example, an approach to achieve balanced particulate flow is to provide pathways or conduits having an equal equivalent length. Equivalent length is defined as a factor which is applied in the calculation of the conduit pressure drop and represents the developed length of the conduit plus the length equivalent for bends, fittings, expansion/contraction of the conduit, etc. In addition to the equivalent length, the total pressure loss in the conduit is determined from a friction factor associated with the conduit internal surface, specific volume of the conveying fluid/particulate matter mixture, and the square of the total conduit mass flux and the conduit diameter, assuming the conduit is circular. The mass flux is the total mass of pressurized fluid and entrained particles which move across a surface that is perpendicular to the outer wall of the conduit.
Restrictors are introduced into conduits to provide the additional pressure loss necessary to achieve a balance in the pressure drop among all conduits when the mass flux is equal. The addition of restrictors has the same effect as the addition of bends etc. in that the addition of a restrictor changes the equivalent length of a conduit.
Applying either of the above techniques cannot achieve a balanced system, i.e., equal distribution of the solids mass flow within each conduit, in a continuous operating regime. Thus, an allowable margin is generally adopted for the acceptance of the final arrangement and this typically results in an imbalanced system, i.e., unequal and varying distribution of the solids mass flow within each conduit. Testing of the system for balance is normally performed with the pneumatic fluid only, at mass flows within the expected operating range and the velocities (or mass flow) measured in each conduit. Typically, a variation in velocity among the conduits of +/−5% of the mean velocity (or mass flow) is considered acceptable, but this variation can result in significant solids mass flow variations between pathways, of up to +/−40% of the mean mass flow. An imbalanced system, even at +/−5% of the mean pneumatic velocity (or mass flow), can have a significant negative impact on the operation of a process that relies on equal distribution of the solids mass flow within each conduit for optimum performance. For example, distribution of coal through conduits providing coal from a coal mill to burners in a coal fired boiler, as previously discussed.
In summary, existing techniques for balancing the flow of particulate matter (solids flow) within a number of conduits are inadequate and compromise the effectiveness or efficiency of the process in which they are installed.
What is needed is a technique that substantially balances the flow of particulate matter (solids flow) within a number of conduits that does not compromise the effectiveness or efficiency of the process in which it is installed.